Changing files in public_html/…/images

I would like to change the text “Read More” to “Leer más” in my WP site with d5 design theme. First I thought to change the text in funtions.php, but I found out that the “Read More” text actually is a button-image (.png). So I would ask you guys: “What if you just change the image (with some Dig. photo editing software) “read-more.png” in the public_html/wp-content/themes/d5-design/images with the symbol and text that you prefer?” Would this action harm in some way the “tecnological structure” (ie. Source code?) of my WP site?