If the images are actually the claimed resolution, then they should be (at least) 3508 x 2048 pixels.
You can verify that with ImageMagick’s identify
command.
ImageMagick’s convert utility should be able to convert the .TIFF files (or GIFs - really?) would certainly be able to turn .GIF or .TIFF into .PNG, which is what you want to use, and unless you tell it to (or you are converting from a vector graphic format) it won’t alter the resolution.
I don’t know how recent it is, but the -format
flag for mogrify
saves a lot of typing:
mogrify -format png *.gif
I’ve tested it on my Ubuntu 22.04 and (despite mogrify normally being modify-in-place) this did produce a new copy of the files in the relevant format. Obviously, having a backup is a sensible precaution, anyway.