Generic as can be:
Image data: (wi, hi) and define ri = wi / hi
Screen resolution: (ws, hs) and define rs = ws / hs
Scaled image dimensions:
rs > ri ? (wi * hs/hi, hs) : (ws, hi * ws/wi)
So for example:
20
|------------------|
10
|---------|
-------------------- --- ---
| | | | 7 |
| | | | | 10
|---------- | --- |
| | |
-------------------- ---
ws = 20
hs = 10
wi = 10
hi = 7
20/10 > 10/7 ==> (wi * hs/hi, hs) = (10 * 10/7, 10) = (100/7, 10) ~ (14.3, 10)
Which as you can see clearly scales to the screen size, because the height is that of the screen but clearly keeps aspect ratio since 14.3/10 ~ 10/7
UPDATE
Center the image as follows:
call (wnew, hnew) the new dimensions.
top = (hs - hnew)/2
left = (ws - wnew)/2