Sigil has an Open with on its Images right click menu - seems to work. Not if you save what was a PNG to a TGA of course, nor if you change the file name from say fred.tif to fred.tiff
But resizing, resampling, scribbling on, blurring etc, etc, are OK. I suppose if one resized an image and text is wrapped around the image then I one might have some work to do.
BR