Inserting a signature image is easy, and the look right and holding concerns are the parts worth doing deliberately, so both:

The insertion, Adobe Acrobat Reader's Fill and Sign, free: the Sign tool lets you add a signature, either drawn, typed or an uploaded image of your actual signature, placed where you want on the page and sized to fit. An uploaded image of a real ink signature on white, ideally with the background made transparent first so it sits cleanly over the document rather than in a white box, looks the most authentic. Save and the signature is part of the document.

The looking right details: a signature image with a transparent background, a PNG rather than a JPG, avoids the white rectangle around it and cropping it close to the ink and sizing it to match where a handwritten signature would naturally sit sells the authenticity. A signature that is too large, boxed in white or floating oddly reads as pasted, while a well prepared transparent PNG placed naturally reads as signed.

The holding question, the important one: a signature image inserted with Fill and Sign is visually placed but not cryptographically bound, meaning someone could technically remove or alter it, since it is an image on a page rather than a secured signature. For casual signing, agreeing to a document, this is fine and universal. For anything where the signature's integrity matters legally, a digital signature is the different and stronger thing: Acrobat's Certificates, Digitally Sign uses a digital ID to cryptographically sign, which both proves who signed and detects any change to the document after signing, the actual answer when tamper evidence matters.

The choice by purpose: signature image via Fill and Sign for everyday signing where a visible signature suffices and everyone acts in good faith, digital signature with a certificate when legal weight, identity proof or tamper detection genuinely matter. Flattening the PDF after placing an image signature, print to a new PDF, at least merges it into the page so it is not a trivially draggable object, a middle step for image signatures that should stay put without full digital signing.