If your logo is inside an editable area on the template, this is the expected behavior. If that's the case, the best way I've found to update a logo is to replace the actual image file with an updated file (same size and file name / extension).
If the logo is inside of a module, the existing module in your email should not get updated, but the copy in the module sidebar should be the updated version once you re-approve an email(s). To get the updated module (with the new logo) into your email (if it's in a module), you'll need to drag in a new copy of the module containing the updated logo and delete the existing logo module. This makes for a bunch of extra steps for each email, so I'd recommend going the "replace image" route instead if you can.