My husband and I have been messing around pretty much all day, and we've finally figured out how to add custom icons to attacks and other rolls. Pardon me if it seems noobish, but it may help new people who'd like to customize their attacks. I looked for hours trying to find a step-by step guide on how to do this, but I could only find topics detailing specific issues; no clear, this-is-exactly-what-you-do-and-it'll-work guide. After triumphing about 3 hours ago (and changing all my icons) I decided to make a quick tutorial with pictures showing how I proceeded. As an example I'll use the bite attack on my character Remi.
I'll assume you have the icon you want to use (you have to make it small and preferably transparent, say a 35X35 transparent png. Our roll template is black so my icon is white.). You can tweak your icon, resize, change color etc. using GIMP, paint.net or any other image editing software (I'd avoid MSPaint!). Upload your icon to an imgur account or any image hosting site at your disposition.
In my case, my icon has been uploaded to imgur.
Now for some step by step stuff:
1. Make sure header images are ticked on in your macro options:

2. Alright, let's take Remi's bite attack:
We have our image, located at http://imgur.com/goY8rhr.png (don't forget that '.png' at the end of the url! I forgot to put it in when I started and it took me an hour to figure out why my image wasn't showing!)
Now to change your attack icon you will use this macro string:
{{header_image=[name](url)}}
The name part can be anything, let's make it 'Bite', and in place of url, let's paste our image location so our macro string becomes:
{{header_image=[Bite](http://imgur.com/goY8rhr.png)}}
3. Now to place this string! We go to the attacks tab of the character sheet and locate Remi's Bite, expand it, than expand the 'Macro Text' link:

Scroll to the end of the text, then paste the macro string with the image url at the end, like so, making sure there is a space between the string you pasted and the previous string:

4. Test your attack!

If you want to do it on skills, it's the exact same thing, simply find the macro text box for the skill, prepare your header image string like we did for the bite attack and paste it at the end of the box like so:

And since you put the request for the new image in the macro field linked to the roll button of the sheet, the macros you make using the attacks will still pull that request for the header image and you should still get the image if you use a macro button.
Happy icon-making!
I'll assume you have the icon you want to use (you have to make it small and preferably transparent, say a 35X35 transparent png. Our roll template is black so my icon is white.). You can tweak your icon, resize, change color etc. using GIMP, paint.net or any other image editing software (I'd avoid MSPaint!). Upload your icon to an imgur account or any image hosting site at your disposition.
In my case, my icon has been uploaded to imgur.
Now for some step by step stuff:
1. Make sure header images are ticked on in your macro options:

2. Alright, let's take Remi's bite attack:
We have our image, located at http://imgur.com/goY8rhr.png (don't forget that '.png' at the end of the url! I forgot to put it in when I started and it took me an hour to figure out why my image wasn't showing!)
Now to change your attack icon you will use this macro string:
{{header_image=[name](url)}}
The name part can be anything, let's make it 'Bite', and in place of url, let's paste our image location so our macro string becomes:
{{header_image=[Bite](http://imgur.com/goY8rhr.png)}}
3. Now to place this string! We go to the attacks tab of the character sheet and locate Remi's Bite, expand it, than expand the 'Macro Text' link:

Scroll to the end of the text, then paste the macro string with the image url at the end, like so, making sure there is a space between the string you pasted and the previous string:

4. Test your attack!

If you want to do it on skills, it's the exact same thing, simply find the macro text box for the skill, prepare your header image string like we did for the bite attack and paste it at the end of the box like so:

And since you put the request for the new image in the macro field linked to the roll button of the sheet, the macros you make using the attacks will still pull that request for the header image and you should still get the image if you use a macro button.
Happy icon-making!