You might like to check out the SimV Help & Tutorials section. http://www.simviation.com/fsdhelp.htm
There is at least one tute on the subject of repainting FS98 aircraft. The tutes for later versions of FS will not be much help. You will need a reliable texture converter. The best I've found for this job is ConvR8 by Martin Wright. Use this with any graphics editor/paint program of your choice. You should find it posted in the Utilities section or visit his own site. http://www.mnwright.btinternet.co.uk/index.htm