For a while now you’ve been able to add borders and crop images right inside the Google Presentations editor, and I’ve also given some workarounds, such as in this post, for using other image manipulations such as transparency.
Google has now added some more image editing tools right within the slides editor, so you can now change colourisation, brightness, contrast and transparency right there in the editor.
To access the new tools, use the toolbar button, the Format menu or right click the selected image and choose Image options … :
A sidebar now opens with the editing options displayed:
Re-colour lets you choose a recoloured image from a matrix of possibilities:
Meanwhile, transparency, brightness and contrast can be tweaked just by grabbing and dragging the handles of the three sliders:
If you don’t like the result, you can reset all your changes using the Reset adjustments button provided.
UPDATE: As of December 2014, people working in Google Docs, as well as slides, now have the ability to adjust the colour, transparency, brightness, and contrast of images.
Select an image in your Google Docs document and click the Image options … button in the toolbar (or right click on the image, or use the Format menu, selecting Image options).
As in the above information for presentations, under Recolour you can change the colour of your image, while under Adjustments you can alter the transparency, brightness, and contrast.