There is a Page drop-down box at the bottom where you can select different Pages to be displayed.
Unfortunately, you can't just display All your comics in one big list.
There is a way to use Advanced find to limit you list to only issues you own that are missing covers but that is a two-step process. Find Issues you own that are missing covers. Select ALL (once for each page worth of comics), the MARK them all (you can't download covers from Advanced Find).
Once Marked, then do a regular Find for all the Marked issues. You still need to do one 'page' of downloads at a time but your list should be much shorted (since it only has Missing cover issues).
The bad part of this is you have to go to all the pages twice (once in Advanced Find to Mark them, and again with the Find for Marked issues.
Depending on how many Issues with Missing Covers you have, it may still take less time doing this twice then doing it once for All your issues.
In Advanced find Enter: SELECT: I.* WHERE: I.[Quantity in Stock] >= 1 and PictureModified < #01/01/1900#
PS It would be best to run Rebinds Lists -> Issue Information with the Rebuild pictures list checked before the Advanced find to ensure picture information is up-to-date.