Your iMac has a couple of disadvantages using for graphic design. It is an i3 which is slower than an i5 or i7, has a 512MB Graphics card which is quite is quite slow compared to later models with 2,4 or even 8GB cards. Also the memory is part of the memory board and cannot be upgraded.
Having said that if you have an SSD installed, and the maximum 16GB memory it would be adequate as long as you use just one program at a time. Before retiring I always used the i7 models for graphic design and publishing with Quark Express.