Sorry I kinda skipped few steps.
You should get a GRF Tool if you haven't already.
LINK[STEP 1] Then, you would want to search the sprite name of card in Korean (there is a little
silver SPR icon on upper right of each item).
이름없는카드 is what you are looking for (<< just copy that to make your life easier).
[STEP 2] With the Korean letters you copied, put it in the search box in GRF Tool and start opening .grf files of you RO client.
[STEP 3] Extract the 이름없는카드.spr file.
Image for the
3 steps:
In this case I used Abysmal Knight Card, but any card would be fine since they have the same sprite name.

After you extract the file, instead of
이름없는카드.spr, you would get
À̸§¾ø´ÂÄ«µå.spr.
Annnnnnnnd after all these,
3. Use Photoshop or equivalent photo editing program and change the ratio of the bmp file
4. Save bmp file with same name
5. Use SPR Conview again and convert bmp to À̸§¾ø´ÂÄ«µå.spr
6. If you open the new .spr file you should be able to see the change if you did everything successfully above
7. Move the new .spr file to (commonly I believe) data\sprite\¾ÆÀÌÅÛ (create new folders if you don't have these)
I will try to explain more clearly if there are more questions :3