how to concatenate an integer with a string

CString is not a standard C++ class, so I would stick to a standard C++ solution:
#include <sstream>
#include <iostream>
using namespace std;
int main()
{
stringstream st;
st<<"C\\:\\MyGallery\\AnImage";//insert the string part to the stream
st<<100; //insert the integer and concatenate it to the previous value
string result;
st>>result; //result=C\:\MyGallery\AnImage100
}

I think CString is part of MFC library (Visual C++) and not BCB (VCL).

Thi scertainly won't work since strcat attempts to append additional bytes at the end of the array somestring. This is a classic buffer overflow.
In addition, (char*)someint does something quite different: it casts the value 100 to a pointer address, 0x0000064 which is an invalid address on most platforms.