Pretty much I don't see a point in using a grid for this. It will give you even more headache than to help you. Since most grids will try to make the elements responsive and will resize them in order to fit them on the same line on smaller width. And since you don't want the images to resize, don't go to responsive grids at all...
You can choose a normal grid but why importing 1000+ lines of code for something that can be done in 1 line.
Pretty much one line of code (if written on one line that is):
ul li {
display: inline-block;
*display: inline;
zoom:1;
vertical-align: top;
width: 50px;
height: 50px;
margin: 0 10px 10px 0;
background-color: #000;
}