Yes, you'll need to store 'i' in some temporary register. As you said $t0 is a good place.
Just keep looping, but branch when 'i' ($t0) is greater than 25. Your loop may look something like:
Code:
addi $t0, $0, 1
addi $t1, $0, 25
loop:
# i <= 25
bgt $t0, $t1, end
# ... stuff in the loop
# i += 2
addi $t0, $t0, 2
# go to the next iteration
j loop
end:
# ... stuff after the loop ...
Don't get caught up in the while/do-while/for loops, they all can be written as for loops, while loops or do-while loops.