Well, I know this isn't easy, and its essentially the "local" getting there - not much faster than the long slog of doing it yourself, but....
I've been working on an application - a front end to a few tagging utilities. I've recently added picture scaling support in, but not in the way you want. It's only for mp3's; only using id3lib; only writes id3v2.4 tags, and should probably only be used on duplicates. And currently, what you want done isn't what I had envisioned it doing, but if you are intrepid, you could adapt it as the source is included (sadly, decent documentation isn't).
http://members.verizon.net/pucklock/id3Infiltr8/IDInfiltr8.htm
However, if you avoid the program altogether, and instead use the embedded command line programs directly using Terminal.app, it can be less of an ordeal. Drag the program onto your desktop and choose "Show package contents" and navigate to "Contents/Resources/Support/id3lib/id3Gateway" - that's the program that will do it. You can copy the id3lib folder onto your desktop, just keep the folder structure inside the "id3lib" folder as is. You can speed things up by skipping typing of the filenames and just use the finder to drag-n-drop icons onto the Terminal window. The path will show up (including a handly space after each drop).
--1. extract pix to destop
Code:
/path/to/id3Gateway /path/to/your.mp3 --extractPix ~/Desktop/outPix
(1 or more pictures appear on your desktop "out-PicType0-num1.jpg" for example)
--2. remove pix in your mp3
Code:
id3Gateway /path/to/your.mp3 --removePix
--3. set the environmental variables to handle your pictures:
--the below environmental variable has to be in the same window you are doing the resizing with id3Gateway in, it's not a "global" variable; life is limited to the window session
--here 150 is the maximum dimensions (it should maintain aspect; double check)
--"MaxKBytes=60" limits jpeg filesize to 60kilobytes; it also maxes out at 30% quality setting
Code:
export PIC_OPTIONS="MaxDimensions=150:DPI=72:MaxKBytes=60"
--or
Code:
export PIC_OPTIONS="MaxDimensions=150"
--4. now set the pictures inside the mp3 file The is a pair of double quotes after each 0 (it's just for "Description" which is a feature of the tag & not supported in iTunes):
Code:
id3Gateway /path/to/your.mp3 --APIC /path/to/picture1.jpg 0 "" --APIC /path/to/pic2.jpg 0 ""
--or if you only have 1:
Code:
id3Gateway /path/to/your.mp3 --APIC /path/to/picture1.jpg 0 ""
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and you should have reduced pictures. It's not what you wanted, but it's not agony either. Keep in mind: it's FREE.