Hello,
I can only copy comment from jaku:
1. you shouls use the relative path; that is "images/logos/logo.gif"; see if the path where TCPDF looks for the file matches the real location (this has to be SYSTEM FILE PATH), not the web (URL) path
2. if you are 100% sure the path is correct, then try with other image; note that TCPDF does not support all types of images (so e.g. animated GIFs or transparency PNGs or with some special color palettes) will not work! There might be also other limitations, so try - just for testing - with more images in more formats (you can also try with JPG);
Did you check image format?