Album Art - File Cannot Be Accessed

I have MP3Tag version 2.48. I load a directory, select all the files, then press Shift+Alt+5 to open up the menu needed. I choose 'import file from folder' to apply an album cover, but when I select it, it says file cannot be accessed. Even when I do it individually, it sometimes fails to work. Help!

Which filename have you specified to look for and which file is present in the folder?
WHich format does the picture have? jpg? png? other?
Does the extension match the cover file format?