When it comes to optimizing graphic assets you want to use texture atlasses (aka sprite-sheets). Since you can have multiple images in one and therefore have less seperated files to open and load one by one and creating expensive textures into your graphic memory. For instance 95 seperate images or one sprite sheet makes a huge difference:

When you want to create sprite-sheets there’s one really awesome tool called Texture Packer.

Which basically creates them by dragging and dropping them into a scene optionally re-orderes, compresses and creates a file with all the offsets for it with like 3 clicks.

At work we’re using it for some time already and I can highly recommend it.

By the way there is one similar cool tool for css sprite sheets called Sprite Cow. :)