On the iPhone and iPad there is no option to drag and drop a bookmarklet, however if you follow these instructions you sure can add one.
Add this page as a bookmark as you would normally.
Select and copy all the text starting with javascript: in the grey box below.
javascript:var unencodedURL = (location.href); var websiteURL = encodeURIComponent(unencodedURL); var url = 'http://api.snapito.com/?url=' + websiteURL + '&type=png&um=box&freshness=1&filename=image.png'; window.open(url, 'Download', '');
Go to your bookmarks and tap "edit". Then tap the new bookmark (Send to Pinterest) you just made of this page.
Press the (x) to remove 'http://snapito.com/iOS-screenshot-bookmarklet.html' and then press and hold, select Paste from the popup. Now the javascript:... text (that you copied from the grey box) should be where 'http://snapito.com/iOS-screenshot-bookmarklet.html' used to be.
That's it