How can you be sure that your application supports a particular file format? Test, test, test...
Files adhering to the written format specification, and software applications that work with them, are called fully conforming, or just conforming. Fully conforming software should always make the conservative choice if there is any ambiguity in the format specification. Of course it's not always clear what conservative means in any specified context, but the point is to not extend the format if given the chance, no matter how tempting the opportunity. In any case, if you write software to manipulate graphics files, you'll need some fully conforming files to test the code.
If you happen to be working with TIFF, GIF, or PCX, files in these formats are often just a phone call away, but beware. Files are not necessarily fully conforming, and there is no central registry of conforming files. In some cases, effort has gone into making sets of canonical test files; for example, some are included in the TIFF distribution.
The trick is to get a wide spectrum of files from a number of sources and to do extensive testing on everything you find--every size, resolution, and variant that can be found. Another strategy is to acquire files created by major applications and test for compatibility. For example, Aldus PageMaker files are often used to test TIFF manipulation software. Unfortunately, PageMaker has historically been the source of undocumented de facto TIFF revisions, and testing compatibility with PageMaker-produced TIFF files does not produce wholly accurate results.
Any number of graphics file conversion programs exist that can do screen captures, convert files, and display graphics and images in many different formats. Once again, you are at the mercy of the diligence of the application author.
Remember, too, that it's your responsibility to locate the latest format revisions. We still see new readers, for instance, on all platforms, which fail to support the latest specification. If you simply cannot locate an example of the format you need, your last resort will always be the format creator or caretaker. While this approach is to be used with discretion, and is by no means guaranteed, it sometimes is your only recourse, particularly if the format is very new.
In any case, don't stop coding and testing simply because one variation of an image file format can be read and converted successfully by your application. Test your application using as many variations of the format, and from as many different sources as possible. Have your application do its best to support the image format fully.