Here is a for loop that generates a different filename in each iteration:
for i = 1:100
newfilename = strcat('Image', num2str(i), '.jpg')
In this example, strcat combines multiple strings into one string and num2str converts a number to string.
Make sure you are imwriting these files in the correct output directory. An easy way to do this is by putting the path to the output directory in which you want to save stuff, using the cd command, before the for loop.