If you resize your pictures to 1024x768 and save them as a .jpg, they are usually small enough for the gallery. Then you just insert the gallery link that appears below the picture in you posts.
When you try to attach an image it can not be larger than 50000 bytes. That can be a small image due to the file size restriction for attachments. I use the gallery most of the time. Just upload them first.
Here is one from my gallery at 1024x768.