Let's analyze the code step by step to understand the CWE-190 (Integer Overflow or Wraparound) vulnerability:

1. Input analysis:
   - The function takes multiple parameters, including pointers (pDstData, pSrcData) and unsigned integers (nXDst, nYDst, nDstStep, dstBytesPerPixel, etc.).

2. Vulnerable line:
   - Line 6: const BYTE* pDstStart = &pDstData[nXDst * dstBytesPerPixel + nYDst * nDstStep];

3. Potential for integer overflow:
   - The calculation involves multiplication and addition of unsigned integers.
   - nXDst * dstBytesPerPixel: This multiplication could overflow if either value is large.
   - nYDst * nDstStep: This multiplication could also overflow if either value is large.
   - The sum of these two products could further overflow.

4. Consequences of overflow:
   - If an overflow occurs, the resulting pointer pDstStart may point to an unexpected memory location.
   - This could lead to accessing or writing to unintended memory areas.

5. Lack of bounds checking:
   - There's no validation of input values to prevent potential overflows.
   - The function assumes the input values will not cause an overflow.

6. Context from freerdp_image_copy function:
   - This function calls overlapping, passing various parameters.
   - There are some checks (e.g., for nHeight and nWidth), but none specifically prevent the overflow in the overlapping function.

Conclusion:
The vulnerability arises from the potential integer overflow in the pointer arithmetic on line 6. Large values of nXDst, dstBytesPerPixel, nYDst, or nDstStep could cause the calculation to wrap around, resulting in an incorrect memory address for pDstStart. This could lead to accessing or modifying unintended memory locations, potentially causing crashes, data corruption, or security vulnerabilities. To fix this, input validation or the use of safe integer arithmetic operations should be implemented to prevent potential overflows.