That would be incredibly cool to be able to resize then optimize images right through the content editor interface. The feature could use all the cool libraries that ImageOptim uses: PNGOUT, Zopfli, Pngcrush, AdvPNG, extended OptiPNG, JpegOptim, MozJPEG (jpegtran & jpegrescan), and Gifsicle. It gives incredible results, ll of them are open source.
https://imageoptim.com/ (I recommend this software. With all the above libraries linked, it's capable of doing things you can't achieve at all with Photoshop, like 8 bits PNGs with an Alpha layer, holy grail of the images for the web)
Also, how cool would it be to have a Webflow-UI version of a library like ImageMagick*?
It's not limited to image resizing and optimization. You can perform a lot of tasks such as creating a canvas, gathering images in it and place them, add layers and control fusion modes, use filters to color them (think about generating avatars or badges). It has plugins and you can even use Fourier transformations. We even developed a gradient map layer for internal use once.
GD and Imagemagick aren't the only way to perfom tasks on images. I stumbled upon this link a few days ago, I had no idea you could perform this kind of manipulations on images using SVG: http://stackoverflow.com/questions/27245489/how-to-mimic-ps-gradient-map-effect-with-svg-fecolormatrix
*GD2 and Imagemagick are among the most popular image manipulation libraries, very well known by whoever worked with CMS like Drupal, Joomla and others. http://www.imagemagick.org/script/index.php