1.Will the engine support bumpmapping and lightmapping later?
No, however if you do create normal maps for your textures, other engines like the Xreal engine can make full use of your textures easily.
OpenArena supports light mapping, though this is calculated when compiling the map, so there is no need to create your own(unless you really wanted too but that's for the mapper to decide.
). If your talking about deluxe mapping the answer would be no.
2.Should the images have a special format or a maximal resolution?
The format generally used is .tga. When creating the source then the highest resolution possible is best, however for the game it will be down scaled. Though all textures must be of the power 2, 512x512, 1024x1024 etc...
3.Anything else i should know?
For the OpenArena project to accept any content it must be under the GPL.
OpenArena also aims to create all the original textures from quake 3.