I usually highlight the photos I want by using my mouse and dragging the cursor across them (they will turn blue) and then uploading them in the usual way. To get rid of huge gaps I click on the page between the photos and press the delete button to delete the spaces in between until the gaps are as big as I want them. Hope this helps:)
Rosie has said exactly what I would have said...good luck!
You can also select the photos you want by clicking on the first one then press shift and at the same time, click on the last one that you want and they will all be highlighted. Then you should be able to upload the lot in one go.