One great tool for converting images to Nokia format ("ota-format" is use LogoManager (www.logomanager.co.uk). LogoManager can open several graphical format (like BMP) and convert it to Nokia format (.otb-files). This tool is good for creating and editing logos.
If you are looking mass conversion software, then you can found LMCvt from same company. It can make mass conversion
Converting picture messages with same software is a little bit complicated, because after conversion, you need to edit it with bin/hex editor a little bit.