You most likely need to resize your photo so it is a much smaller file size. Most photo viewing programs have this capability.
Regarding sashing, it is challenging to sash the way you describe. The challenge is keeping the blocks lined up on either side of the long sashing strip. Use a ruler and mark the wrong side of the sashing where the blocks are already attached, marking the side of the sashing where the new row of blocks connect. Pin (a lot) or glue to keep them lined up as you piece it. I prefer to use cornerstones but there are situations where you want an uninterrupted sashing. Even in those cases I will often have seams in the sashing and sash the blocks as I go. Peckish posted a great example of how to do that a while back. Hopefully she sees this post and will repost her technique. If you still want a long sashing strip, take your time, mark and pin and you should be ok.