1. If you want to save picture in temp file, which will be open using Image in VFP, you do not need to save picture with correct extension. You can save, for instance, GIF file as "TempFile.jpg" and Image will open the file properly;
2. Anyway, if you want to know properly type of picture, consider adding description field in table, which will contain, in addition to other information about picture, also file type. Some formats contain file type (for instance, GIF files begin with used GIF format (GIF87a, GIF89a) - you can see this by opening some GIF file in Notepad; also JPEG files contain JFIF string near to beginning of file; BMP files begin with "BM"). But some formats do not contain this information (ICO, CUR).