Sizing of photos

Hello all,hope everyone had a great Thanksgiving if you celebrate.
Would like to ask if someone could tell me where to go for help,I need photos to be converted using Gif.Jpg.or Png.My site use to work with photos being ICO 256x256,now it will not except this,it says I need to change to one of the three I have mentioned(Gif.Jpg.or Png).If someone could help me figure out a size that would be exactly the same I would appreciate that.I used to convert my photos to ICO 256x256 and it was always perfect,now I am lost.If anyone can help me in a way that is really easy that would be great.I have been looking high and low for the easiest fix for this problem.
Thank you,have a great day.

Could you post up the exact message you get. Omeka generally uses imagemagick to produce derivative images, and that generally works with many different formats beyond the ones you listed.