Capturing image from window

Use a picture control. It is what it was designed for, and more importantly, it has an auto-refresh option, which the image control doesn't.

The Stretchblt and BitBlt api calls both allow you to choose the starting x and y position andthe x width and the y width of the window you want to copy from. When pasting to a picture box, set auto-redraw to true and pass it the picture1.hdc. Then when the copy is done, do a picture1.refresh. Otherwise it won't draw.
If you have autoredraw set to false, it will simply draw the image on the window itself and not in the picture box's buffer. This means that it won't refresh the image for you when the window is obscured/unobscured.

Declare Function BitBlt Lib "gdi32" (ByVal hDestDC As Long, _
ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal _
nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal dwRop As Long) As Long
Declare Function GetDC Lib "user32" (ByVal hwnd As Long) As Long
Declare Function GetClientRect& Lib "user32" (ByVal hwnd As Long, lpRect As RECT)
Declare Function GetWindowRect& Lib "user32" (ByVal hwnd As Long, lpRect As RECT)
Declare Function StretchBlt& Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, _
ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As _
Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal nSrcWidth As Long, ByVal _
nSrcHeight As Long, ByVal dwRop As Long)
Private Const SRCCOPY = &HCC0020
Type RECT
left As Long
top As Long
right As Long
bottom As Long
End Type

This will copy from one window to another.
Call BitBlt(form1.picture1.hdc, 0, 0, XEnd - XStart, YEnd - YStart, SrcWindowHDC, XStart, YStart, SRCCOPY)
form1.picture1.refresh

This will copy from one window to another and resize to the total client area of the destination window.
Dim DestRect As RECT
Call GetClientRect(DestHwnd, DestRect)
Call StretchBlt(form1.picture1.hdc, 0, 0, DestRect.right - DestRect.left + 1, DestRect.bottom - DestRect.top + 1, SrcHDC, srcXStart, srcYStart, srcXwidth, srcYwidth, SRCCOPY)
If you don't know the hdc of the window you want to copy from, get it's handle, then
hdc = getdc(handle)

Once again, don't use this method to get the picture box's device context. Use it's .hdc property or you will end up drawing directly on the window.
form1.picture1.refresh