If you upload to this site (using that yellow box with the mountain in it), your pics will ALWAYS come out as thumbnails in your post. I try to reduce the size of my pics from the camera size to something smaller. A JPG uploaded to this site is limited to 145.5K and for me that is something over 1200x900 with medium compression.
You can play with resizing using one of several different tools on your computer, and use WINDOWS Explorer to look at the file directory and see the sizes you are getting. Then, when someone clicks on that thumbnail, and the pic comes up FULL SIZE it will be as large as possible, as it should be displaying at the full size you uploaded it at after clicking on the thumbnail.
There are suggestions on resizing pics in two threads found in the "Sticky for Stickies", found
HERE.
I frequently re-use pics I have previously uploaded. In that case, they will not be thumbnails, but rather, full size pics, loaded from the GJ server. If you go to "USER CP" in the menu bar at the top of any page, you will find an item at the very bottom left of that screen, "Attachments" and if you look you will find EVERY pic you have ever uploaded (unless you deleted it) and the thread and the post it is in.
When I want to re-use a pic, I simply find it in that list of pics (15 pages of them for me) and right click on the pic link and "copy link location" and then go back to my post in progress and click on that mountain on the yellow picture icon and then right click, paste the link in and click OK to finish it out. The image then appears in the post full size as if it were loaded from an external source, photobucket, etc.
I have two tabs open, both with GJ showing, to do this, one the User CP/attachments screen and one the post I am composing.
Charles