function ImSaveImageToFile(FileName: PAnsiChar; const Image: TImageData): Boolean; cdecl;

Modified to

function ImSaveImageToFile(FileName: PAnsiChar; const Image: PImageData): Boolean; cdecl;
or
function ImSaveImageToFile(FileName: PAnsiChar; var Image: TImageData): Boolean; cdecl;

Delphi Pass a  PImageData pointer to Lazarus
or
var Image

run  is OK  ;D

Delphi create Image:　TImageData;
ImNewImage(256, 256, ifA8R8G8B8, Image);

Passed to Lazarus
Run " TestImage(Image) "

Result is:
LongInt(Image.Width)  1242488        // Should be 256
LongInt(Image.Height)  1243380       //Should be 256
LongInt(Image.Format)  6244311      // Should be 88

Google to Similar problems：
http://community.freepascal.org:10000/bboards/message?message_id=335641&forum_id=24082

Lazarus1.0.2 Compiled  "VampyreImaging.lpi"
Generate "VampyreImaging.dll"

VC++ Run "test.c"
........................
/* save image to Targa format */
ImSaveImageToFile("ctestimage.png", &Img);  //Unable to generate images
DelphiXE3 Call VampyreImaging.dll //Unable to generate images too

but VampyreImaging.dll of DelphiXE3  Generated
generate images is OK.

How do I set Lazarus？

Sorry, my English is poor。  :-[