Samuel

$ gdalwarp.exe -ts 256 256 -of PNG -r bilinear "utm32.tif" "d:/thumb_gdal.png"

Output:
Output driver `PNG' not recognised or does not support
direct output file creation.  The following format drivers are configured
and support direct output:
  VRT: Virtual Raster
  GTiff: GeoTIFF
  NITF: National Imagery Transmission Format
  HFA: Erdas Imagine Images (.img)
  ELAS: ELAS
  MEM: In Memory Raster
  BMP: MS Windows Device Independent Bitmap
  PCIDSK: PCIDSK Database File
  ILWIS: ILWIS Raster Map
  SGI: SGI Image File Format 1.0
  Leveller: Leveller heightfield
  Terragen: Terragen heightfield
  ERS: ERMapper .ers Labelled
  RMF: Raster Matrix Format
  RST: Idrisi Raster A.1
  INGR: Intergraph Raster
  GSBG: Golden Software Binary Grid (.grd)
  PNM: Portable Pixmap Format (netpbm)
  ENVI: ENVI .hdr Labelled
  EHdr: ESRI .hdr Labelled
  PAux: PCI .aux Labelled
  MFF: Vexcel MFF Raster
  MFF2: Vexcel MFF2 (HKV) Raster
  BT: VTP .bt (Binary Terrain) 1.3 Format
  IDA: Image Data and Analysis
  ADRG: ARC Digitized Raster Graphics
  SAGA: SAGA GIS Binary Grid (.sdat)