I don't know how Chrome plugins work. I assumed you knew that part.
I don't understand how you can add buttons to an existing web page just using the plugin. Does it somehow insert new stuff into the existing HTML???
Sorry. You'll have to figure that part out.
Annotating my code:
// get a collection of *ALL* <img> tags on the page
var images = document.getElementsByTagName("img");
// initialize our "found" variable to indicate not found
var found = false; // or null
// this is the image name (src=".../xxxx.jpg") that you want to look for
// we use a regular exprssion because
// (1) the ending $ means that you want to find xxxx.jpg at the end of the src= attribute
// (2) the /i on the end means "ignore case"
var lookFor = /xxxx.jpg$/i;
// so look through all the <img> tags on the page
for ( var i = 0; i < images.length; ++i )
// if our regular expression matches the .src of any <img>...
if ( lookFor.test( images[i].src )
// change our "found" flag to indicate we found a match
found = true; // or found = images[i] if you want a reference to it
// ...and exit the for loop