This is using one of the frames from this site. :D
http://farm3.static.flickr.com/2064/2046944669_c306292f69_b.jpg

Looks like you are off to a good start. If you are looking for background ideas... why not a web.

looking good... But maybe a Web on the back like Gravital said...

Good start. Nice to see You can use the frames.
However - front and back don't play so good together yet. I realize that this is early wip - so try to keep in mind that taking some of the blue colour from the front and incorperate it in your work on the back will give a more balanced look.
Perhaps a black/blue bottom and then the said web could do the trick for ya?

Great ideas guys! Anyone know how I can insert a web? I guess a bacground pic or wallpaper maybe? And a blue gradiant......

I agree with the web idea. Can't wait to see what you do with this next.

Yes a wallpaper could be one method. Another is to take a look at some of the opening credits and design it off of that. I know that sounds a little intense but hey thats part of doing customs. There may be a web pattern that you can grab off the web it would just take a little lookin around.

Still looking for a web but I went ahead and did the whole set. Kind of like a theme. I just wish I could find a darker Spiderman image for the cover of 2. But, since each movie has it's own phrase out back, the second one seems to work with "A Hero fights for us" Kind of like Spidey's real "outing" What do you guys think?
http://farm3.static.flickr.com/2136/2051297841_8b5ec02545_b.jpg
http://farm3.static.flickr.com/2197/2052068696_d6d08a74f6_b.jpg
http://farm3.static.flickr.com/2020/2050267339_264cbb9ee1_b.jpg

Looks good to me joe. I think you should make the title treatment on the first Spiderman spine the same size as the others so that it looks nice when all three are aligned.

On the 2nd cover. Copy your front pic image. cut out spiderman. desaturate your pic of the city - it will go black n white. Now you have a darker front. (You may have to readjust the org. pic (ctrl+l) and set black level at about 30).

Along with what has already been covered also take some time to clean up the edges on the front images. The templat ethat you are using has corner portions and little slits that need to have the image removed from them.