Derramando comes from the verb derramar, which means "to spill". It's a much more poetic and dramatic way to describe crying than the common verb llorar (to cry).
In the song, Selena sings "tantas lágrimas que estoy derramando" (so many tears that I am shedding). This creates a powerful image of her tears uncontrollably spilling out, showing the depth of her sadness over a lost love and a fading photo. It's a beautiful word for expressing overwhelming emotion.
Holding onto a physical picture after a breakup hits differently, and Selena gets straight to the point in "Fotos Y Recuerdos" (Photos and Memories). The whole track revolves around a single photograph from the night the two met at a party: "De aquella fiesta en que te conocí / De ese tímido beso que te di" (From that party where I met you / From that shy kiss I gave you). She kisses it every single night before bed ("Que beso cada noche antes de dormir"), but there is a messy reality to it: the paper is literally falling apart.
She admits the photo is "media rota" (half torn) and "ya se está borrando / Por tantas lágrimas que estoy derramando" (already fading from so many tears I'm shedding). It is a dramatic, bittersweet scene. Even though crying all over an old picture wears the ink away, that ruined piece of paper is literally all that remains of the relationship: "Y es todo lo que me queda de tu amor / Sólo fotos y recuerdos" (And that is all I have left of your love / Only photos and memories).