I'm going to show you step by step how to make a GIF from video.
I first saved the Mario video Heather sent out in an email, but you can use any video file.
Open Photoshop and go to File >Import >Video Frames to Layers...
Once you select the video you'd like to use, this box will open up. Select the part of the video you'd like to use in your GIF. It is best to choose a small section as it will be a smaller size file. Also, it's a good idea to click the box for "Limit to every 2 frames". This will also make the file smaller and easier to edit.
You want to have the Timeline tab open (which you can open from Window > Timeline) to look at all your frames. From here you can readjust the duration on each frame.
You can Shift select multiple frames and click the small down arrow to change it to a set time or choose Other... and put in your own time. 1.0 is long while 0.05 is very short. I'm going to keep them at their preset delay, which is 0.04.
I'm going to make the size of the image smaller because it's rather bulky the way it is now. Go to Image >Image Size. I'm going to change the width to 600 pixels, keeping the same proportions. Now that it's smaller and easier to deal with, I'm going to save it as is.
First of all, save this as a regular Photoshop PSD file. You might want to edit some things later, and it's always good to have a back up. Next, you want to hit File >Save for Web. This window will pop up in Photoshop. It gives you a bunch of information about this file that might seem overwhelming to you right now. The most important thing here is the Animation Looping Options. Make sure it is set for "Forever". Then you can hit Save... and name it whatever you like.
This is the GIF as is. I didn't mess with the layers or add any new content.
Now it's time for part two: adding drawings on top of the GIF!
I've got my PSD file open with layers and timeline still on. I've made a new layer (which I titled "new") and drew a blue line. You can draw or add whatever you like to this GIF. I want to draw this blue line going over several of the frames so it looks like it's moving along with the GIF, so I'm going to make a new layer over every frame I want to add it to. I always make a new layer, just in case I mess up and can't fix the layer back to its original state.
As you can see, in the timeline, the new layer has shown up as it's own frame. We don't want that, but we don't need to worry about that right now. If you want to see your drawing from the previous layer, simply adjust the opacity of the image in between or click the eye to turn off visibility.
Now I've gone and drawn a bunch of blue lines and merged them to the layer I wanted them to be on. NOTE! Make sure visibility is on when you merge down, or else you might merge to the wrong layer! Now the timeline and the animation of the GIF gets a little wonky when you edit the layers like this. To fix this and to be safe, I select all the frames in the timeline with Shift + select and hit the trashcan icon. DO NOT delete your layers, only the frames in the timeline! You're going to be left with one single frame. After that, you want to go to the small drop down box in the upper right corner of the timeline tab and select "Make Frames From Layers".

Now that all those are new frames, they have been set to 0 seconds as their delay. We want to change that back to 0.04 seconds like our first GIF. Shift + select all the frames in the timeline and click the small down arrow next to the delay time. Since 0.04 seconds isn't in the preset list, you will want to click "Other..." and type in 0.04. You can preview the animation in the timeline or change the duration of the frames as you like.
It's time to get the GIF onto the web, so just like before, you want to go to File >Save for Web and hit Save. Make sure the animation loop is on Forever!
It's not even that easy to notice, but I did draw a few frames of a blue squiggly line in here. You probably want to save this file as it's own PSD as well if you want to go back and edit it again. It's never too safe to save more than one version!!!