Yeah I get that problem now and then myself. Lucky for me I have photoshop! If u use photo shop you can open up the image, then go to the "edit" window and scroll down to "Mode" (i think it's called mode
) another scroll window will come up, select the "Bitmap" option. It will come up asking for the color options, choose "Windows (256 color). Careful though...if ur tile sets background is something like green it may mess it up and cause you to have two different colored pixels which would be appearent in RPG Maker o_0. It's better to use a blue or white background for this. Next save the image as a PNG file, and if an option comes up asking if it should interlace the image choose "None." Provided the file is the right size, RPG Maker should accept it now. Sometimes saving as a bitmap can work, but i find it more successful to save as PNG...
If you don't have Photoshop I think you can download a trail version from the internet, which should be good for at least 30 days...hope this post is of some use.