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The Dropbox iPad app can upload images from the camera roll to your Dropbox account. You could then access your Dropbox account from within Textastic.

You'd probably still need to edit your image (resize, crop or whatever) somehow though.
Of course, just tap on the blue arrow icon to the right of each connection to modify the connection's settings.
Please make sure to move your fingers over the selection handles to grab them. Textastic currently expects a pan gesture to drag the handles. If you just do a long press on a selection handle the regular loupe appears instead.

Does this work for you?

Note: I just tried this out in regular text fields and you are right, Apple's edit controls accept a long press gesture on the selection handles. I will change this behaviour in an update.
I plan to implement something similar to this idea: http://feedback.textasticapp.com/feedback/24460-4-way-direction-arrow-keyset-brought-up-with-gesture/

"a simple gesture (two finger swipe, or something) to bring up a floating keyset with the 4 direction arrows"
You can delete remote files. Just select the files you want to delete and tap on the right-most button in the toolbar, then choose "Delete Selected".

Here's a screenshot illustrating this: http://www.textasticapp.com/support/delete_remote_files.png

You can also rename files by tapping on the "pen" button that is to the right of each file.
It works when the XML file has the correct styling tags and the CSS file is written correctly.
In Textastic 2.1 you can scroll the additional row of keyboards horizontally. If you move the row to the left you can get to the semicolon and other keys.
Unfortunately, I haven't received the files yet. Update: got it now and will report back :)