gallery macro: Support predefined/variable image sizes

Dear all,

the gallery macro is great, but it lacks some features that our users mostly requested. Some of them are already listed in CONF-4778.
The requests below

• Support a (initial) size parameter for the image slidshow. This should also resize the popup accordingly.

Background: Mostly uploaded are images from digital cameras, 3 Megapixels and up. Those images do not fit on the screen, so you have to scroll the image left/right up/down which is annoying. Also, the images (e.g. 1-3 Megabytes) are transferred over the Network as a whole, where one could save traffic when the size was smaller. Unfortunately, the users are not that much trained so they could upload resized images (e.g. 800x600) -> The server must do the work.

• Introduce a size combobox in the slideshow

The slideshow could have a combobox where the user selects the image size (e.g. 800x600, 1024x768, original size). Then, the image is scaled down by the server and transmitted. Saves network traffic and each user could select the size that fits on screen.
Optionally, each tumbnail and the slideshow picture could get its own "Save original picture as..." link, so each user can download the original picture easily to get high-quality printouts.

What do you think?

Thanks for considering
Andreas