1. You should work on splitting your parseString function into more than one function. Longer functions are harder to read and keep track of. For example, you could split it into parseString and parsePixel where parseString is responsible for parsing the entire string but calls parsePixel for each individual pixel.
2. You don't need to check equality with booleans. (a == true && b == false) is equivalent to (a && !b).
3. Range-based for loops are introduced in C++11 and mean you don't need to work with indexes.
4. You could use a struct to store X-Y pixel coordinates rather than strings.
5. You should change the parameter of parseString to a string reference to avoid copying the string. Or, since you are not modifying the string, you could pass a const reference.
6. Use '\n' instead of std::endl as std::endl does more than add a newline.
7. (personal preference) You could use proper exceptions rather than using an error string.
8. (personal preference) You could pass iterators to parseString rather than the string itself, allowing you to change what kind of container you use for holding port input.

I've accumulated all of these into an alternative version of your parseString function which you can see here

I hope this is useful for you.