Digital signatures vs. signature images: what's the difference
Last updated: 2026-07-27
When people talk about signing a PDF, they are usually describing one of two completely different things. The first is placing a picture of a signature onto a page. The second is applying a cryptographic digital signature backed by a certificate. Both produce a file you might describe as signed, but what is happening inside them has almost nothing in common.
One thing up front: this is general information about how the technology works, not legal advice. Whether a particular signing method is sufficient depends on your jurisdiction, the type of document, the agreement between the parties, and sector-specific rules. For anything consequential, run it past counsel.
A signature image is a picture
Technically, adding a signature image means positioning a graphic on top of a page. You sign a sheet of paper, scan it, save a PNG, place it over the signature block, and export a new PDF. The result looks much like a scanned paper contract, which is exactly why the workflow caught on.
The thing to understand is that this picture has no mathematical relationship to the document. The file does not record who placed it or when, and it cannot tell you whether the person who applied the image is the person it depicts. Nor can it tell you whether anything changed afterward — if someone edits a payment term and re-applies the same image, the file gives no indication. The image can also be extracted and reused on an unrelated document, which is why signature graphics deserve to be treated as sensitive files in their own right.
That does not make the approach useless. For internal approvals, delivery confirmations, and other low-risk documents between parties who already trust each other, it is far more practical than printing, signing, and rescanning. StickPDF does exactly this — places an image onto a PDF page and produces a new file — entirely in your browser, so the document is not uploaded anywhere. What it does not do, and what no image-placement tool can do, is make a document legally valid or tamper-evident.
A cryptographic digital signature is a different mechanism entirely
Certificate-based digital signatures do not involve pictures at all. At signing time, the software computes a hash of the document — a fixed-length fingerprint of its exact contents — and encrypts that hash with a private key only the signer holds. The resulting signature value, along with the signer's certificate, is embedded in the PDF.
Two properties fall out of this. First, tamper evidence: change one character anywhere and the hash changes completely, so verification fails and the viewer reports that the document was modified after signing. Second, identity binding: the certificate is issued by an authority that verified the holder to some standard, and only the holder of the matching private key could have produced that signature value.
A common misconception: a properly digitally signed PDF may show no handwriting on screen at all. The visual appearance is optional decoration; the real signature is data inside the file. Conversely, seeing cursive script on a page tells you nothing about whether a cryptographic signature exists. You have to open the viewer's signature panel and check the validation status.
How the law frames this, at a high level
In the European Union, eIDAS defines three tiers. A simple electronic signature is broadly any electronic data attached to a document and used to sign — typing your name, clicking an accept button, or pasting a signature image can qualify. An advanced electronic signature must be uniquely linked to the signer, capable of identifying them, created using means under their sole control, and linked to the data so that later changes are detectable — which in practice requires cryptography. A qualified electronic signature is an advanced signature made with a qualified signature creation device and a qualified certificate, and it carries legal effect equivalent to a handwritten signature across the EU. Crucially, eIDAS says a signature may not be denied legal effect merely for being electronic or for not being qualified. That is not the same as saying all three tiers are equally persuasive in a dispute.
In the United States, the federal ESIGN Act and the state-level UETA take a similarly technology-neutral approach: a contract or signature may not be denied enforceability solely because it is electronic. The emphasis is on intent to sign, attribution to a person, association with the record, and retention of an accurate record. Neither mandates a particular technology, and both carve out categories — wills, certain family law matters, some notices — where electronic execution is limited or excluded.
The practical takeaway from both frameworks is the same: legal recognition and evidentiary strength are different questions. If a counterparty later claims they never signed, a certificate-based signature offers an objective verification result. A pasted image leaves you arguing from email trails and circumstances.
Choosing in practice
Three questions usually settle it. What does the receiving party or authority actually require? If a format is specified, the discussion is over. How likely is this document to be disputed, and what is at stake if it is? And do internal policy or sector regulations — financial services, healthcare, public procurement — dictate a method?
For routine internal paperwork and confirmations with trusted partners, an image stamp is often entirely adequate and vastly faster. Where tamper evidence or signer authentication genuinely matters, use a certificate-based signing service or an established e-signature platform. Mixing both across different document classes is completely normal.
Whichever you use, look after the signature image file itself. And once more: this is a general explanation of the technology, not legal advice — the specifics for your situation belong to a qualified professional.