I don't see much of a question here, but you are right. It is especially visible if processing a JPG image with a medium compression ratio -- the 8x8 squares of which JPG's are built of tend to get more visible after filtering.
This is comparable to the rising of computational error when operating on computer-based floating point numbers.
Your best bet for image processing is using lossless formats for image processing -- PNG's are a good choice, cause they both provide lossless compression, as well as a decent support for bitdepths, transparency and are browser-compatible.
Another format, more often used in the professional world are TIFF's (Targa).
However, note that if your source image is already in a loss-based format, converting it to a lossless one will only prevent adding additional artifact's, not spreading and enhancing the old one. You can however reduce the extent of error by converting it to a lossless format and running it through a small seed gaussian blur.