Second preimage mostly, which is harder than collision with most common algorithms (even MD5 might still be secure for this, not that anyone should use it for a

Second preimage mostly, which is harder than collision with most common algorithms (even MD5 might still be secure for this, not that anyone should use it for anything at this point). Collision resistance is only important if someone can create two versions at the same time that have the same hash and wants to quietly switch them later. Using SHA-256 as you describe works well and is widely used by package systems to ensure that source code being built does not change. Signatures can be better for determining if the initial image is valid if the signing is done away from the distribution infrastructure since development machines might be more secure than distribution infrastructure (and if not you will have problems either way). You still need to get the correct public key to start with. However, if you do have a good image by luck or actually secure enough distribution infrastructure then SHA-256 will certainly let you know that you get the same one later. Signatures almost always sign a cryptographic hash of the message and not the message itself.