I have pictures of Bricks/Stones and Clouds I would like to share these with you at some point. 
 
Now I understand seamless should mean that the edges of the picture should line up with other edges to make them seamless, but this is not always possible is there a  trick to make them seamless?? and the ones that I have with clouds in them... if the edges are simply the blue sky is that good enough to be considered seamless??

now i remember a thread that had a tutorial on just such a subject, from some time ago - i think it involved mirroring the negative image over the texture to make the shadows cancel out or something - off to have a look........

If you have Photoshop, you may want to try this free plug-in.http://www.redfieldplugins.com/filterSeamlessWorkshop.htm.
personally i use a nifty wee program that makes seamless textures from any picture at the click of a mouse, but since i am at work and i have the old brain-fog just now ,i cannot for the life of me remember its name. 
also heres a neat tutorial on How to Create a Seamless Texture using Adobe Photoshop.
tutorial (By Darren Hester)
 i will try to remember that programs name and post it later.

Easy way to clean up seams in images:

• use filters/other/offset to offset your image
• use the "spot healing brush tool" to clean the seams (left col, 4th icon from top)
• repeat the offset filter to find remaining seams
• repeat this untill ur happy

Tip: Use little strokes with the healing brush. It'll give better results. Also the brush favours its sample location in the direction where your stroke ends.

enjoy!

Got a simple step by step tut on how to create a seamless texture from a photo of a brick wall - 
http://www.sharecg.com/v/6488/tutorial/Creating-a-Brick-Texture
The actual texture used, plus the result, are also in my portfolio there.