先引用delphi自带的单元
uses EncdDecd;

然后就可以使用下面二个函数了:

///将Bitmap位图转化为base64字符串
function BitmapToString(img:TBitmap):string ;
var
ms:TMemoryStream;
ss:TStringStream;
s:string;
begin

ms := TMemoryStream.Create;
img.SaveToStream(ms);
ss := TStringStream.Create('');
ms.Position:=0;
EncodeStream(ms,ss);//将内存流编码为base64字符流
s:=ss.DataString;
ms.Free;
ss.Free;
result:=s; 

end;

///将base64字符串转化为Bitmap位图
function StringToBitmap(imgStr:string):TBitmap;
var ss:TStringStream;

ms:TMemoryStream;
bitmap:TBitmap;

begin

ss := TStringStream.Create(imgStr);
ms := TMemoryStream.Create;
DecodeStream(ss,ms);//将base64字符流还原为内存流
ms.Position:=0;
bitmap := TBitmap.Create;
bitmap.LoadFromStream(ms);
ss.Free;
ms.Free;
result :=bitmap;

end;