If you want to study graphic design, then you need to have a portfolio with examples of your work in it, such as the logos. However, if you don’t have a lot of graphics work to put in, surely you have other artwork that can show your potential as an artist. I would add those in also.
Graphic design includes more than logos. It can include setting up a page, say for a magazine ad, or for a PowerPoint presentation, choosing colors to send a particular message, or simply showing that you have an eye for design. Your portfolio is supposed to show what you have done in the past and give a glimpse of what you could do in the future if you get the training the school is offering. So make sure you put all of your best examples of your work, whether it is drawing, painting, photography or logos.