I used Blender to export a 3D model in X3D format. However, textures are automatically exported in TGA format. In order to print multicolor models in Shapeways (https://www.shapeways.com), the requirement is to provide textures in JPG or GIF format. Is there any way to do this in Blender or any other tool?
In the UV/image Editor select the texture or image you want to convert and press F3 (or Image->Save As),
Then select which format you want to save as.
(If you don't see these options on the left side of the screen press T.)